Swift - Complete action after NSURLSession resumes

So I am downloading several images from my server with the url, which all works good. But now I am trying to save those images to the disk after I have finished downloading them.

Problem

Now I can save them inside the same query while downloading them but I would prefer not too as it makes my query slower.

So I have added a completion handler to my func with the query, but when I save the images to the disk in that block it works but I cannot do anything with my screen as the query has not resumed yet it is blocked from touches I guess...

Now I would like to be able to call my func to save the images to the disk straight after the query has been resumed.... Anyone have any idea?

If someone needs more explanation or to see code just drop a comment below